The present disclosure relates generally to electrolyte solutions that can be used for electropolishing articles made from metals, and in particular, for electropolishing metallic medical devices (e.g., stents, closure devices, and the like) made of stainless steel, titanium, tungsten, nickel-titanium, tantalum, cobalt-chromium-tungsten, tantalum-nickel-tungsten, etc. While the electrolyte solutions described herein are mainly applicable to metallic medical devices, the disclosure is not limited to such medical devices. For example, the methods may be applied to electropolish metallic automotive or aerospace components.
Electropolishing is an electrochemical process by which some of the surface metal is electrolytically dissolved. In general, the metal article (e.g., a stent) is connected to an anode and connected to a power supply while immersed in an electrolyte solution. A metal cathode connected to the negative terminal of the power supply is also included in the electrolyte solution. Metal is removed from the anode surface by the action of the current and the electrolyte solution as current flows from the metal article (as the anode) to the cathode. The rate at which metal is dissolved from the metal article is controlled, at least in part, by the applied current and/or voltage, the positioning of the cathode relative to the metal articles, and/or distribution of the electrolyte around the article. According to the theory of electropolishing, the current density is highest at high points protruding from a surface and is lowest at the surface low points. Thus, the higher current density at the raised points causes the metal to dissolve faster at these points which thus levels the surface.
Stents are generally tube-shaped intravascular devices placed within a blood vessel to maintain the patency of the vessel and, in some cases, to reduce the development of restenosis. Stents may be formed in a variety of configurations which are typically expandable since they are delivered in a compressed form to the desired site. Example stent designs include, but are not limited to, helically wound wire, wire mesh, weaved wire, serpentine stent, a chain of rings, or laser cut tubular stents. The walls of stents are typically perforated in a framework design of wire-like connected elements or struts or in a weave design of cross-threaded wire. Some stents are made of more than one material. The stent may be, for example, a sandwich of metals having outer layers of a biocompatible material, such as stainless steel, with an inner layer providing the radioopacity to the stent needed for tracking by imaging devices during placement. In forming such stents from metal, a roughened outer surface of the stent may result from the manufacturing process (e.g., from processes such as tube drawing and laser cutting).
It is desirable for the surface of the stent to be smooth so that it can be easily inserted and traversed with low friction through the blood vessels toward the site of implantation. In addition, a rough outer surface may also damage the lining of the vessel wall during insertion. Furthermore, smooth surfaces decrease the probability of thrombogenesis and corrosion. Likewise, stents having a smooth, mirror-like finish generally have a better fatigue life because surface defects (scratches, burrs, inclusions, and the like) can be sites for crack propagation.
Since the processing to form metallic stents often results in a product initially having undesirable burrs, sharp ends or debris and slag material from melting the metal during processing, mechanical cleaning (e.g., interior and exterior grinding), chemical cleaning (e.g., descaling), or the like are generally performed. Following cleaning, further surface treatment such as electropolishing is generally performed. Electropolishing is able to provide a mirror-like, defect-free surface to the metal article (e.g., the stent).